1. Field of the Invention
The present invention relates to a cooking apparatus whereby optimum control of a cooking process can be automatically achieved, in accordance with factors such as an amount of food that is to be cooked, a specific degree of hardness of the finally cooked food, etc.
The invention moreover relates to a cooking apparatus whereby a degree of hardness of the food resulting from the cooking process can be freely adjusted to a desired value by the user.
2. Description of the Related Art
Although the present invention is applicable to various types of cooking apparatus, for use with various types of food material, embodiments of the invention will be described which are designed for cooking rice by heating the rice together with water.
With a usual prior art type of rice cooking apparatus, heating power for the cooking process is generated by a heater element, with the level of heating power being controlled by a switch (such as a thyristor), which is alternately set in a conducting and non-conducting state for supplying and cutting off a supply voltage to the heater element. The conduction factor of the heater element (i.e. the ratio of the duration of each interval for which the switch is set in the ON state to the period of successive OFF-to-ON state transitions) determines the level of heating power that is generated by the heater element, and the value of conduction factor that is to be utilized in the cooking process is determined as follows. The rice cooking apparatus is provided with a temperature sensor for detecting the temperature of the food material. Firstly, the conduction factor is set to 100% in the first part of a cooking interval in which the food material (i.e. mixture of rice and water, in this case) is rapidly heated to be brought to the boiling point, and is thereafter maintained at the boiling point for a certain time. That cooking interval, extending from the start of the rapid temperature increase, will be referred to in the following as the boiling interval. During that first part of the boiling interval, the conduction factor is held at 100% until the food material attains the boiling temperature. Thereafter, the conduction factor is set to a value that is determined as follows. Designating the value of conduction factor that is to be used thereafter in the cooking process as .alpha., and designating the temperature gradient of the food material prior to reaching the boiling temperature as .theta. (where temperature gradient here signifies the rate of change of temperature of the food material with respect to time), the value of is obtained from the following linear equation: EQU .alpha.=a.times..theta.+b (1)
Where "a" is a negative constant, and "b" is a positive constant.
The value determined for the temperature gradient provides an estimate of the quantity of food material that is to be cooked. That is to say, the smaller the amount of food material, the greater will be the value of the temperature gradient, and conversely the greater the amount of food material the smaller will be the value of temperature gradient.
With another type of prior art rice cooking apparatus, the apparatus includes a microcomputer, which controls the apparatus to execute a cooking process that has been previously judged by the manufacturer as being optimum. Thus the cooking is fixedly executed in accordance with that process.
However in the case of a rice cooking apparatus which utilizes the method of equation (1) above, the result obtained from such a linear equation is only theoretically valid. In actual practice, such linear relationships do not exist in such a cooking process, due to the effects of such factors as the characteristic of the temperature sensor that is used to obtain the temperature gradient, etc. Thus it has not been possible for a prior art rice cooking apparatus to set the conduction factor of the heater element such as to reliably provide optimum results.
In the case of a rice cooking apparatus which is controlled by a microcomputer in accordance with a predetermined program, since the operation is fixedly predetermined, it will not be capable of compensating for the effects of variable factors such as variations in the amount of food that is to be cooked, etc.
Moreover, with certain types of cooking apparatus such as a rice cooking apparatus, it is desirable to have a capability for cooking the food to a desired degree of hardness, in accordance with the particular requirements of different users. However with a prior art rice cooker, that can only be achieved through the user changing the ratio of the amount of water to the amount of rice, prior to the start of the cooking process. In addition to being inconvenient, such a method has the disadvantage that it is difficult for the user to accurately correctly judge the required amount of water that must be used, in order to achieve the desired degree of hardness in the finally cooked rice.
Although the above prior art problems have been described only for the case of a rice cooking apparatus, such problems will also arise with other types of cooking apparatus, which operate on the basis of fixedly predetermined linear equations or computer programs, such as a cooking apparatus for cooking stewed meat, for example.